Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. In the imaging field, for example, CMOS image sensors have proved to be less expensive to manufacture than CCD imaging devices. Moreover, for certain applications, CMOS devices are superior in performance. Furthermore, the necessary signal processing logic on a CMOS image sensor can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand-alone imaging device. In other applications, however, CCD imaging devices still have some advantages.
Image sensors are used to produce an image representing an object. The primary building block for an image formed by an image sensor is a pixel. Image sensors typically include a two-dimensional array of pixels arranged in rows and columns. The number, size, and spacing of the pixels determine the resolution of the image generated by the imaging device. The pixels of an image sensor are semiconductor devices that transform incident light photons into current signals. The photosignals are proportional to the amount of light reflected from an object to be imaged.
In the array, pixels belonging to the same column (also referred to as bit line) are usually connected at a common output node. At the output node, a column readout circuit reads out and amplifies the photosignals from each pixel in the individual column. Typically, the pixels are read out row-by-row by asserting a row select signal to each pixel of the selected row. After the photosignals are read out, the signals are processed by signal processing circuitry to create an image representing the object.
Personal digital assistants (PDAs), digital cameras, cell phones, and other portable electronic devices often include camera units with image sensors to capture images. These mobile devices include a battery to power the image sensor and other components of the device. Although CMOS imager sensors consume less power than CCD image sensors, both types of image sensors drain the battery. Power consumption is an issue in portable electronic devices because the charge in a typical battery lasts for a relatively short time. To recharge the battery, the device is generally coupled to an AC outlet or another power source. As such, the mobility of the device is reduced during charging. Accordingly, a need exists to reduce the frequency with which the battery of a mobile device must be recharged.